What is the initial incident The possibility of evil?

In "The Possibility of Evil" by Shirley Jackson, the initial incident occurs when Miss Strangeworth, a seemingly sweet elderly woman, discovers a letter that she believes reveals the moral failings of her fellow townspeople. This prompts her to write anonymous letters to them, warning of their "evils." Her actions are driven by a desire to protect her community, yet they ultimately lead to unintended consequences that reveal the darker side of her character and the town. This sets the stage for the story's exploration of perception, morality, and the hidden complexities of human nature.